2b. CHEMICAL BONDING: What is an ionic formula? How do you work out an ionic formula? and how do you name ionic compounds?

(1) A note on explaining what an ionic formula is

For covalent compounds you have a definite formula of a molecule e.g. H₂O meaning two atoms of hydrogen combined with one atom of oxygen to form a single particle of three atoms.

However, ionic compounds form a giant ionic lattice of layer after layer of oppositely charged ions e.g.

(a) For sodium chloride NaCl you have a three dimensional array of positive sodium ions Na⁺ and negative chloride ions Cl^-. There are no NaCl molecules.

Therefore an ionic compound formula gives you the ratio of the component ions.

For example: sodium chloride, formula NaCl means a ratio of one Na⁺ ion to one Cl^- ion and so you would write the ionic formula of sodium chloride as Na⁺Cl^–.

(b) Magnesium bromide, formula MgBr₂ means a ratio of one Mg²⁺ ions to two Br^– ions

So, you would write the ionic formula of magnesium bromide as Mg²⁺(Br^–)₂

Note the need for ()  to avoid incorrectly writing the formula as Mg²⁺Br^–₂

(c) Aluminium fluoride, formula AlF₃ means a ratio of one Al³⁺ ion to three F^- ions.

So, you would write the ionic formula of aluminium fluoride as Al³⁺(F^–)₃

Unfortunately the names of ionic compounds don't usually include prefixes or suffixes to help you work out the numbers of ions in a formula.

So you need to know the charges on the common ions of groups 1, 2, 6 and 7, and this has been explained in detail in the preceding section.

If you know how to work out the charge on ions from the periodic table, you can then work out the correct 'ratio' formula.

There is section near the end on showing you examples of working out ionic formula.

Note that the formulae you deduce are empirical formula, the simplest whole number ratio of atoms in the formulae.

Table (2) Selected ions and charges

In the electrically balanced stable formula, the total positive ionic charge must equal the total negative ionic charge i.e. the total charge is zero.

To work out an ionic formula by combining ion 'A' with ion 'B' the rule is:

number of ion 'A' x charge of ion 'A' = number of ion 'B' x charge of ion 'B'

(you ignore charge sign as long as you realise that A and B have opposite charges)

In doing so, you are working out the ratio of ions in their giant ionic lattice.

Example: How do we work out that the formula of aluminium oxide is Al₂O₃?

As difficult an example as any you will have to work out!

Aluminium oxide consists of aluminium ions Al³⁺ and oxide ions O^2– 

number of Al³⁺ ions x charge on Al³⁺ balances the number of  O^2– ions x charge on O^2– 

the simplest numbers are 2 of Al³⁺ x 3 balances 3 of  O^2– x 2 (total 6+ balances total 6–)

so the simplest whole number formula for aluminium oxide is Al₂O₃ 

More examples of working out the empirical formula of an ionic compound

Reminder:

By applying similar logic you can work out the charge on one ion, knowing the formula and charge on the other ion

e.g. supposing a metal M forms an ionic chloride compound MCl₂, Cl will be the chloride ion (charge single –), so to balance the two chloride ions, the metal ion must carry a charge of 2+ i.e. the M²⁺ ion.

If a metal that forms a singly charged positive ion M⁺, forms an ionic sulfide compound M₂S, the charge on the sulfur ion must be 2– i.e. S^2–, to balance the two + charges of the metal ion.

numerically ion charge = valency of A or B to deduce the formula

valency or ionic charge = the combining power of the ion (see Table 1a above)

'molecular' or ionic style of formula and compound name

 

1 of K⁺ balances 1 of Br^– because 1 x 1 = 1 x 1

gives KBr or K⁺Br^–  potassium bromide

 

2 of Na⁺ balances 1 of O^2– because 2 x 1 = 1 x 2

gives Na₂O or (Na⁺)₂O^2–  sodium oxide

 

1 of Mg²⁺ balances 2 of Cl^– because 1 x 2 = 2 x 1

gives MgCl₂ or Mg²⁺(Cl^–)₂  magnesium chloride

 

1 of Fe³⁺ balances 3 of F^– because 1 x 3 = 3 x 1

gives FeF₃ or   Fe³⁺(F^–)₃  iron(III) fluoride

 

1 of Ca²⁺ balances 2 of NO₃^– because 1 x 2 = 2 x 1

gives Ca(NO₃)₂ or Ca²⁺(NO₃^–)₂  calcium nitrate

 

2 of Fe³⁺ balances 3 of SO₄^2– since 2 x 3 = 3 x 2

gives Fe₂(SO₄)₃ or (Fe³⁺)₂(SO₄^2–)₃  iron(III) sulfate

 

1 of Mg²⁺ is balanced by 2 OH^–

giving Mg(OH)₂  magnesium hydroxide

 

2 of K⁺ is balanced by 1 CO₃^2–

giving K₂CO₃ potassium carbonate

 

1 of Ca²⁺ is balanced by 2 of I^–

giving CaI₂ calcium iodide

 

1 Mg²⁺ is balanced by 2 NO₃^–

giving Mg(NO₃)₂ magnesium nitrate

 

1 Zn²⁺ is balanced by 1 SO₄^2–

giving ZnSO₄ zinc sulfate

 

I hope that is enough examples of working out, naming and writing out an ionic formua!

(3) Working out a formula from periodic table patterns.

positive cation↓ negative anion →	chloride ion Cl–	bromide ion Br–	oxide ion O2–
sodium ion Na+	sodium chloride NaCl	sodium bromide NaBr	sodium oxide Na2O
potassium ion K+	potassium chloride KCl	potassium bromide KBr	potassium oxide K2O
magnesium ion Mg2+	magnesium chloride MgCl2	magnesium bromide MgBr2	magnesium oxide MgO
calcium ion Ca2+	calcium chloride CaCl2	calcium bromide CaBr2	Calcium oxide CaO

Note the simple periodic table pattern of formulae.

See section (4) for more on naming ionic compounds

and (2) How to work out ionic formula (this section assumes you can i.e. total positive cation charge equals total negative anion charge).

(4) Appendix 2 How do you name simple ionic compounds?

How to name ionic compounds: Naming simple ionic compounds isn't difficult and through your course you get used to the names as they crop up.

The name usually consists of two parts, first the name of the positive metal ion, and secondly, the name of the negative ion derived from the non–metal.

For the metal ions (cations) it couldn't be easier, its just the metal name itself e.g. sodium, magnesium, aluminium etc.

For the Na⁺ ion e.g. sodium chloride

For the Mg²⁺ ion e.g. magnesium bromide

For the Al³⁺ ion eg. aluminium oxide etc.

However, there is one complication when a metal can form two different ions like copper or iron.

In these cases the two ions are distinguished in the name by a Roman numerals number in brackets after the name of the metal which corresponds to the numerical value of the positive charge on the metal ion.

e.g. Cu₂O copper(I) oxide contains the Cu⁺ copper(I) ion,

and CuO copper(II) oxide contains the Cu²⁺ copper(II) ion

and FeCl₂ iron(II) chloride contains the Fe²⁺ iron(II) ion,

and iron(III) chloride FeCl₃ contains the Fe³⁺ iron(III) ion.

However, things are a bit more complicated for the negative ions (anions) because although the name of the ion is derived from the name of the non–metallic element, it is a bit different.

Oxygen forms the oxide ion, sulfur the sulfide ion etc. the names of these anions from group 6 end in ..ide,

e.g. sodium oxide, magnesium oxide, aluminum oxide

Fluorine forms the fluoride ion, chlorine the chloride ion, bromine the bromide ion and iodine the iodide ion.

The names of the anions from group 7 halogens (naming ending ...ine) end in ...ide, the halide ions),

e.g. potassium fluoride, sodium chloride, calcium bromide etc.

The name only ends in ide, when the metal is only combined with one other non–metal element e.g. oxygen forming an oxide, sulfur forming a sulfide, other 'ide's include nitrides from nitrogen and phosphides from phosphorus.

For anions where two or more non–metallic atoms are combined in a single ion, and one of them is oxygen, the name often ends in ..ate (NOT oxide)

e.g. carbonate (C + O, CO₃),    sulfate (S + O, SO₄),

nitrate (N + O, NO₃),   chlorate (Cl + O, ClO_x x varies) etc.

see their formula in the table 1a. of Appendix 1 above.